The invention relates to determining the mass fraction burned and cylinder pressure in an internal combustion engine.
Mass fraction burned and cylinder pressure are important parameters in characterizing the combustion process in a cylinder of an internal combustion engine. Traditionally, the mass fraction burned has been determined from the measured cylinder pressure based on the Rassweiler-Withrow Procedure established in 1938. In this procedure, the beginning and end of the combustion process are identified by examining the relationship between cylinder pressure (P) and volume (V) plotted on logarithmic scales, as shown in FIG. 12. The relationship between log P and log V during compression and expansion, in the absence of combustion, is linear. Therefore, the beginning and end of combustion can be determined by identifying the points 900 and 910 where the log P to log V relationship departs from the linear characteristic during compression and expansion. Once the beginning 900 and the end 910 of the combustion process are determined, the mass fraction burned during combustion can be evaluated based on pressure and volume through the following equation: EQU x.sub.b =(P.sup.1/n V-P.sub.0.sup.1/n V.sub.0)/(P.sub.f.sup.1/n V.sub.f -P.sub.0.sup.1/n V.sub.0)
where the subscripts 0 and f indicate the beginning and the end of the combustion process, respectively, and n is a polytropic exponent. The polytropic exponent may change throughout the combustion process and from cycle to cycle in an unpredictable manner, which may affect the reliability and applicability of the Rassweiler-Withrow approach. In addition, this approach requires a pressure sensor in the cylinder.